Recent structural and ultrastructural investigations have estimated that more than 90% of the neurons in the caudate nucleus are intrinsic or local circuit neurons, that is their dendrites and axons terminate within the caudate and interconnect with other neurons in the caudate. Our electrophysiological recordings of single unit responses of caudate neurons to electrical pulses closely applied within the intact or isolated caudate confirm the above anatomical investigations and demonstrate that the population of caudate neurons establishing synaptic connections with other caudate neurons is very large. Although the dopaminergic identity of the nigro-neostriatal pathway has been established, little is known about the neurotransmitter identity of intrinsic caudate neurons. We are investigating the responsiveness of receptor sites of caudate neurons to various putative neurotransmitter substances such as dopamine, norepinephrine, acetylcholine, serotonin, and gamma-amino-butyric acid. This research is conducted in rats and young cats in which one of the two caudate nuclei has been chronically isolated from the rest of the neuroaxis. At a second acute stage, single unitary recordings of the previously isolated and of the intact caudates are taken by means of five-barreled glass micropipettes. We characterize unitary responses to: a) electrical pulses delivered to closely opposed tungsten microelectrodes according to a procedure developed in earlier work, b) to distant (cortex, thalamus, substantia nigra) stimulating electrodes, and c) responses to iontophoresis of putative neurotransimitter substances delivered to the immediate vicinity of the recorded units.